The present invention relates generally to demodulators for digital communication and particularly to demodulator design for modulation schemes that employ in-phase and quadrature components.
In the context of digital communication, demodulators must recover data which has been transmitted by modulating a generally sinusoidal signal. Demodulator design is specific to the modulation scheme employed. In a typical application, the input to a demodulator is a modulated signal received via a transmission medium and downconverted by well-known mixing techniques to a fixed intermediate frequency (IF). The demodulator output is a stream of symbols which represent estimates of the transmitted data.
Error correction techniques as well as data decompression may be applied to the demodulator output but certain error sources such as channel echoing and accompanying inter-symbol interference must be at least partially removed by the demodulator. Furthermore, the demodulator must synchronize its operation to the transmitted data.
Newly adopted digital television standards envision the application of QAM (Quadrature Amplitude Modulation) to the commercial broadcast of high-resolution video images over cable transmission media for reception by consumer equipment. The standards anticipate data rates commensurate with commercial cable channel bandwidths, yet the necessary QAM demodulators must be sufficiently inexpensive for incorporation into consumer receiver equipment.